Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. The total particulate matter (“TPM”) emissions are comprised of three main components. One component is the carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust.
A second component of the TPM is the soluble organic fraction (“SOF”). The soluble organic fraction is sometimes referred to as the volatile organic fraction (“VOF”), which terminology will be used herein. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust, and is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulates is the so-called sulfate fraction. The sulfate fraction is formed from small quantities of sulfur components present in the diesel fuel. Small proportions of SO3 are formed during combustion of the diesel, which in turn combines rapidly with water in the exhaust to form sulfuric acid. The sulfuric acid collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of TPM.
One key aftertreatment technology in current use for high level particulate reduction is the diesel particulate filter. There are many known filter structures that can be used to remove particulates from diesel exhaust, including honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters that are described below have received the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust. The filter is a physical structure for removing particles from exhaust. As particles accumulate on the filter, an increase in the back pressure from the filter on the engine arises. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. Unfortunately, the carbon soot particles require temperatures in excess of 500 to 550° C. to be combusted under oxygen rich (lean) exhaust conditions. This is a higher temperature than is typical of diesel exhaust temperatures. Provisions are generally introduced to lower the soot burning temperature in order to provide for passive regeneration of the filter. The presence of a catalyst has been found to promote soot combustion and thereby regeneration of the filters at temperatures accessible within the diesel engine's exhaust under realistic duty cycles. In this way a catalyzed soot filter (CSF) or catalyzed diesel particulate filter (CDPF) can be an effective method to provide for >90% particulate reduction along with passive burn-out of the accumulating soot, and thereby filter regeneration.
A frequent problem that plagues use of less porous CSFs is caused by too rapid a build up of the particulate matter on the filter, particularly at lower diesel exhaust temperatures. Lower diesel exhaust temperatures are observed, for example, under conditions of low load and at startup. At lower diesel exhaust temperatures, the catalysts disposed on the filter are less effective at catalyzing the combustion of the particulate matter collected on the filter, and back pressure in the exhaust system increases. Various strategies have been implemented in the past that seek to address buildup of particulate on CSFs and subsequent buildup of back pressure, and are described below.
U.S. Pat. No. 4,902,487, for example, discloses a process for treatment of diesel exhaust wherein the exhaust is passed through a filter to remove particulate therefrom before discharge, and the particulate deposited on the filter is combusted with a gas containing nitrous oxide (NO2) that is catalytically generated in the exhaust stream. Putting a catalyst upstream of the filter is disclosed to accomplish this. The upstream catalyst is disclosed to produce NO2 in a diesel exhaust stream, which contains nitric oxide (NO).
GB 1,014,498 discloses methods and apparatus for filtering and purifying internal combustion engine exhaust containing solid particles. The speed of the exhaust is progressively reduced by passing the gases radially outward through a first filter of a pellet type oxidation catalyst adapted to retain the larger of the particles, and then radially outward through a second filter surrounding the first filter and adapted to retain smaller sized particles. The first filter is composed of pellets that are refractory and attrition resistant and that are impregnated with active catalyst substance (e.g., alumina grains impregnated by cobalt oxide (Co2O4) having a diameter of 1.5 to 5 mm). The second filter is composed of synthetic fibers although other materials can be used to form the second filter.
U.S. Pat. No. 4,510,265 describes a self-cleaning diesel exhaust particulate filter which contains a catalyst mixture of a platinum group metal and silver vanadate, the presence of which is disclosed to lower the temperature at which ignition and incineration of the particulate matter is initiated. Filters are disclosed to include thin porous walled honeycombs (monoliths) or foamed structures through which the exhaust gases pass with a minimum pressure drop. Disclosed useful filters are made from ceramics, generally crystalline, glass ceramics, glasses, metals, cements, resins or organic polymers, papers, textile fabrics and combinations thereof.
U.S. Pat. No. 4,426,320 (“the '320 patent”) discloses a method whereby carbon and lead particles are removed from internal combustion exhaust by passing the gases through a course filter and then through a fine filter. The use of a relatively coarse filter in front of a fine filter is said to allow the coarse filter to remove larger particles before the gases reach the second fine filter, and also to extend the life of the fine filter. The filter configuration is described to reduce the rate at which back pressure increases as the particles accumulate in the filters. Deposited on the filters are catalyst materials effective for conversion of one or more pollutants in the exhaust to innocuous entities and removing suspended particles in the gas. A preferred filter is disclosed to be an open cell ceramic foam filter.
Catalyst materials disclosed in the '320 patent that are suitable for the combustion of carbon particles include an element of the first transition series such as vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc. Compositions disclosed in the '320 patent are also useful in conversion of hydrocarbons, carbon-monoxide and nitrogen oxide pollutants. Such catalyst materials are said to have a noble metal, an element of the first transition series, and mixtures thereof.
The '320 patent describes the deposition of the catalyst materials directly on the ceramic foam supports by means of (1) impregnating the filter with a water-soluble, thermally decomposable inorganic salt or complex of the metal or metals; (2) drying the impregnated filter; and (3) then calcining the dried filter. Alternatively, the catalyst materials can be supported on a porous, refractory inorganic oxide.
U.S. Pat. No. 4,535,588 (“the '588 patent”) discloses the use of a carbon particulate cleaning device that has an embodiment wherein two different filters having different air permeability with each other are serially arranged. Filters of stainless steel wool are described, with the filter of higher air permeability being positioned at the upstream side of the second filter of relatively lower air permeability. In alternative embodiments, ceramic or metallic foam filters, having different air-permeability, replace the stainless steel wool filters. The filters can be coated with oxidizing catalysts, such as platinum, palladium and rhodium.
U.S. Pat. No. 4,828,807 (“the '807 patent”) discloses the use of serially arranged filter elements installed in the cross section of a housing traversed by the exhaust, wherein at least one filter element carrying a catalyst that lowers the ignition temperature of the soot and assists in soot burn off alternates several times with at least one filter element carrying a catalyst that assists in the combustion of gaseous pollutants. Filter elements that can be used include filtered disks sintered into open porosity, disks from pressed ceramic fibers, particularly fibers of Al2O3, SiO2, aluminum silicate, or ZrO2; disks formed from sintered metal; disks formed from pressed steel wool; packed beds of temperature resistant ceramic or metallic material. A number of catalysts are disclosed in the '807 patent which assist in the ignition and burn off of the particulates. These catalysts include (a) lithium oxide; (b) vanadium pentoxide; (c) vanadium pentoxide plus an oxide of one or more of 37 disclosed elements, which elements include lanthanum, cerium, praseodymium, neodymium and zirconium; (d) vanadate of one or more of the metals listed under (c); and (e) perrhenate, preferably of lithium, potassium, silver or vanadium. The catalysts are disclosed to be combined with a carrier material which can be MgO, Al2O3, CeO2, SnO2, TiO2, ZrO2, HfO2, ThO2, Nb2O5, WO3, magnesium silicate, aluminum silicate and/or magnesium titanate and combinations thereof.
The '807 patent also describes catalysts that assist in the combustion of gaseous pollutants which are coated on at least one of the filter elements to include one or more elements of the platinum group, optionally together with one or more base metals in combination with a temperature resistant carrier material. Preferred carrier materials include MgO, Al2O3, particularly γ-Al2O3, CeO2, SiO2, SnO2, TiO2, ZrO2, HfO2, ThO2, Nb2O5, WO3, magnesium silicate, aluminum silicate and/or magnesium titanate or combinations thereof. The carrier material is mixed either with the catalyst or applied to the filter element, and serves as a base for the catalyst.